Drop centering device

ABSTRACT

A method for centering a drop of liquid on a given point on a surface. The inventive method consists in forming a bell-mouthed recess, whereby said recess has a curvature at any point at the limit of contact between the drop and the recess that is lower than or opposite to the curvature of a circle (TC) that is tangent to the surface of the recess to said point and at a symmetrical point (CP 2 ) of said surface.

This application is a continuation/divisional of copending applicationSer. No. 10/812,307, filed Mar. 30, 2004, which is acontinuation/divisional of application Ser. No. 09/937,508, filed May22, 2002, which was filed as a national stage application ofInternational Application No. PCT/FR00/00751, filed Mar. 24, 2000, whichclaims the right of priority based on French Patent Application No.99/03980, filed Mar. 26, 1999. This application is also being filed as acontinuation-in-part of International Application No. PCT/FR98/02143,which was filed Oct. 7, 1998, which was copending with InternationalApplication No. PCT/FR00/00751, and is also being filed as acontinuation-in-part of application Ser. No. 09/529,193, which was filedJul. 25, 2000, as a § 371 national stage application of InternationalApplication No. PCT/FR98/02143 and which was also copending withInternational Application No. PCT/FR00/00751. application Ser. No.09/529,193, and its corresponding International Application No.PCT/FR98/02143, claim the right of priority based on French PatentApplication No. 97/12781, filed Oct. 8, 1997, the former now issued asU.S. Pat. No. 6,369,954.

The present invention relates to the maintaining of a liquid drop in apredetermined position on a solid surface, and more specifically to thecentering of such a drop.

For various reasons, it may be desired to maintain a drop laid on asurface, accurately centered on a predetermined axis. A known way (bythe inventor) consists of modifying around this axis the surfacewetability with respect to the drop liquid.

FIG. 1 shows a side cross-section view of a liquid drop 2 laid on asurface 4. Surface 4 has been processed in a circular area Cl centeredon an axis O. The processing of area Cl is such that its wetability withrespect to the liquid of drop 2 is strong. Thus, capillarity forcesmaintain drop 2 centered on axis O.

An improvement (of the inventor), not shown, of such a surfaceprocessing consists of creating around axis O several concentriccircular areas. The processing of these areas then is such that thecloser an area is to axis O, the more its wetability with respect to theliquid of drop 2 is strong. Such an improvement enables centering drop 2for different contact angles. This is particularly advantageous when away of changing said angle is available, for example by means of anelectric voltage. Such surface processings may however be difficult andexpensive to implement.

An object of the present invention is to center a drop laid on a surfacewithout using a modification of the surface wetability.

To achieve this object, the present invention provides a method forcentering a liquid drop at a given location of a surface, which consistsof forming at this location a flared hollow such that, at any point ofthe contact limit between the drop and the hollow, said hollow has acurvature smaller than or opposite to that of a circle tangent to thehollow surface at said point and at a symmetrical point of this surface.

According to an embodiment of the present invention, the flared hollowhas the shape of a truncated cone with an axis perpendicular to saidsurface.

According to an embodiment of the present invention, the flared hollowhas the shape of the upper central portion of a torus having an axisperpendicular to the surface.

According to an embodiment of the present invention, a method forcentering a liquid drop on the external surface of a convex surface isprovided, which consists of giving this surface at any point of thecontact limit with the drop a shape such that this surface has acurvature greater than that of a circle tangent to this surface at thispoint and at a symmetrical point of this surface.

According to an embodiment of the present invention, the convex surfaceis formed by revolution against said axis of an arc of a circle having aradius smaller than that of said tangent circle.

The present invention also provides a variable-focus lens implementingthe above-mentioned method, which includes a wall made of an isolatingmaterial, a drop of a first isolating liquid arranged on an area of afirst surface of the wall, a second conductive liquid covering the firstsurface and the drop, the first and second liquids being non-miscible,having different optical indexes and substantially the same density, andmeans for applying an electric voltage between the conductive liquid andan electrode arranged on the second surface of said wall, the drop beingplaced in a flared hollow of the wall.

According to an embodiment of the present invention, the electrode is asheet metal, the flared hollow is a truncated cone formed by embossingsaid sheet metal, centered on an axis perpendicular to the firstsurface, and the bottom of which is pierced with a centered hole, andthe isolating material wall is a transparent plastic film flattenedagainst the electrode and the walls of the hollow, and which covers saidhole.

According to an embodiment of the present invention, the electrode is asheet metal, the flared hollow is a truncated cone formed by machiningsaid sheet metal, centered on an axis perpendicular to the firstsurface, and the bottom of which is pierced with a centered hole, andthe isolating material wall is a transparent plastic film flattenedagainst the electrode and the walls of the hollow, and which covers saidhole.

The foregoing objects, features and advantages of the present invention,will be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

FIG. 1, previously described, shows a side cross-section view of adrop-centering means according to prior work of the present inventor;

FIG. 2 shows a side cross-section view of a liquid drop laid on a planarsurface;

FIGS. 3 to 7 are side cross-section views of a liquid drop placed in aflared hollow of a surface;

FIGS. 8 to 10 are side cross-section views of a liquid drop located atone end of a drop-grasping rod;

FIG. 11 shows a side cross-section view of an application of the presentinvention to the centering of a transparent liquid drop used as a lens;and

FIG. 12 shows a side cross-section view of an alternative of FIG. 11.

Same elements have been designated with same references in the followingdrawings. In particular, reference 2 will designate a liquid drop ofsmall dimensions, having its position on a surface essentially definedby the capillarity forces (the surface tension). Reference 4 willdesignate a surface, the wetability of which is constant with respect tothe liquid of drop 2.

FIG. 2 shows a planar surface 4 cut by an axis O perpendicular to thissurface. If a liquid drop is desired to be placed at a position Acentered on axis O, for example, by being deposited from a drop-graspingrod, the chances of achieving this with precision are small, the dropremaining where it has been laid, for example, at a position B.

As illustrated in FIG. 3, to solve the problem of the centering of adrop 2 with respect to an axis O, the present inventor has first thoughtof placing drop 2 in a hollow formed in surface 4, this hollow havingthe shape of a spherical cap symmetrical with respect to axis O.However, any position of drop 2 in hollow 6 has appeared to be a stableposition. Thus, as in the case of a planar surface 4, if drop 2 isdesired to be placed at a position A centered on axis O, the chances ofachieving this with precision are small since the drop will remain whereit has been laid, for example, at a position B.

French patent application number 97/12781 (corresponding to U.S. Pat.No. 6,369,954) deposited by the present applicant describes avariable-focus liquid lens formed by a liquid drop laid at the surfaceof a solid, centered on an axis by the means described in relation withFIG. 1. This application further describes electromagnetic means fordeforming the drop while maintaining it centered on the axis. On theother hand, this patent application describes a variable-focus liquidlens formed by a liquid drop contained in a tube and maintained centeredon the tube axis by capillarity. There thus appears that a drop 2 can becentered on an axis O by being placed in a cylindrical hollow of surface4, centered on this axis. However, it is difficult to place a liquiddrop in such a cylindrical hollow while avoiding for a bubble to remainat the drop basis.

First, the present inventor has thus abandoned the idea of centering adrop by means of a hollow. Thus, French patent application number97/12781 deposited by the present applicant describes a variable-focusliquid lens formed by a liquid drop laid at the surface of a solid,centered on an axis by the means described in relation with FIG. 1. Thisapplication further describes electromagnetic means for deforming thedrop while maintaining it centered on the axis.

On the other hand, this patent application describes a variable-focusliquid lens formed by a liquid drop contained in a tube and maintainedcentered on the tube axis by capillarity. There thus appears that a drop2 can be centered on an axis O by being placed in a cylindrical hollowof surface 4, centered on this axis. However, it is difficult to place aliquid drop in such a cylindrical hollow while avoiding for a bubble toremain at the drop basis.

The present inventor has then systematically searched which type ofhollow centered on an axis O enables easy centering of a liquid drop onthis axis.

FIGS. 4 to 7 each represent a side cross-section view of a liquid drop 2placed in a flared hollow 6 centered on an axis O and formed in asurface 4. For a position A of drop 2, centered on axis O, CP1designates any point of the contact limit between drop 2 and the surfaceof hollow 6. The circle having its center located on axis O, and whichis tangent to the surface of the hollow both at point CP1 and at asymmetrical point CP2, is called the tangent circle TC.

FIG. 4 represents a liquid drop 2 placed in a hollow 6 formed by therevolution around axis O of an arc of a circle having a radius smallerthan that of previous circle TC. Thus, at any point CP1 of the contactlimit between the hollow surface and the drop, the surface curvature ofthe hollow is greater than that of circle TC.

FIG. 5 shows a liquid drop 2 placed in a hollow 6 formed by revolutionof a segment around axis O. Hollow 6 is a truncated cone. Thus, at anypoint CP1 of the contact limit with the drop, the surface curvature ofhollow 6 is null, smaller than that of tangent circle TC.

FIG. 6 shows a liquid drop 2 placed in a hollow 6 formed by revolutionaround axis O of an arc of a circle greater than that of previous circleTC. Thus, at any point CP1 of the contact limit with the drop, thesurface curvature of the hollow is smaller than that of tangent circleTC.

FIG. 7 shows a liquid drop 2 placed in a hollow 6 formed by revolutionaround axis O of an arc of a circle having a curvature opposite to thatof tangent circle TC. Hollow 6 corresponds to the central upper portionof a torus. At any point CP1 of the contact limit with the drop, thesurface curvature of hollow 6 is opposite to that of tangent circle TC.

The present inventor has shown that, for a drop 2 placed in a flaredhollow 6 centered on an axis O, the surface curvature of hollow 6 at anypoint CP1 of the contact limit with the drop determines whether aposition A of the drop centered on axis O is a position of equilibriumor not.

Thus, when, as in FIGS. 5, 6, and 7, the curvature at any point CP1 issmaller than or inverse to that of tangent circle TC, a drop placed insuch a hollow naturally takes a position A centered on axis O.

On the other hand, when, as in FIG. 4, the curvature at any point CP1 isgreater than that of tangent circle TC, a position A of the drop,centered on axis O, is particularly unstable and will not be able to bemaintained. A drop placed in such a hollow naturally takes anout-of-center position B with respect to axis O.

Finally, when, as in FIG. 3, the curvature at any point CP1 is equal tothat of tangent circle TC, any position of the drop is stable, and adrop placed in such a hollow at a position B brought out of center willkeep this position.

Thus, the present invention provides a method for centering a drop at agiven location of a surface consisting of forming at this location aflared hollow such that, at any point CP1 of the contact limit with thedrop, this hollow has a curvature smaller than or opposite to that oftangent circle TC.

It should be noted that only the contact limit between the drop and thehollow surface counts. The shape of the hollow has no importance and itmay be flat, convex, or concave.

According to another aspect of the present invention, a convex surfaceof uniform wetability centered on an axis O is considered, on which islaid a liquid drop, in a position A centered on axis O. Thepreviously-described point CP1 and tangent circle TC are consideredagain.

FIGS. 8 to 10 are side cross-section views of a liquid drop 14 placed ona convex surface 16 centered on an axis O.

FIG. 8 shows the case where the convex surface is spherical. The surfaceof sphere 16 at any point CP1 of the contact limit with drop 14 isalways confounded with tangent circle TC.

In the case of FIG. 9, the convex surface is conical. The “curvature” ofconical surface 16 at any point CP1 of the contact limit with drop 14 isalways smaller than that of tangent circle TC.

In the case of FIG. 10, the convex surface is formed by revolutionaround axis O of an arc of a circle having a radius smaller than that oftangent circle TC. The surface curvature at any point CP1 of the contactlimit with drop 14 is always greater than that of tangent circle TC.

The present inventor has shown that, for a drop 14 placed on a convexsurface 16 centered on an axis O, the curvature at any point CP1 of thecontact limit with the drop determines whether a position A of the drop,centered on axis O, is a position of equilibrium of not.

Thus, when, as in FIG. 10, the curvature at any point CP1 is greaterthan that of tangent circle TC, a drop placed on such a surfacenaturally takes a position A centered on axis O.

However, as in FIG. 9, the curvature at any point CP1 is smaller thanthat of tangent circle TC, a position A of the drop, centered on axis O,is unstable. A drop placed on such a surface naturally takes a positionB brought out of center with respect to axis O.

Finally, when, as in FIG. 8, the curvature at any point CP1 is equal tothat of tangent circle TC, any position of the drop is stable, and adrop placed at an out-of-center position B will keep this position.

Thus, the present invention provides a method for centering a drop on aconvex surface which consists, at any point CP1 of the contact limitwith the drop, of giving the surface a curvature greater than that oftangent circle TC.

According to an application of this method, convex surface 16 forms theend of a rod 18 for grasping a drop 14. Indeed, for certain chemical orbiological handling operations, it is desirable to have drop-graspingrods which enable precisely and reliably handling liquid drops. Adrop-grasping rod, the end of which is formed according to the presentinvention, enables conveying definitely centered drops.

An application of the present invention to the forming of a means forcentering a liquid drop used as an optical lens will now be described.

FIG. 11 shows a simplified cross-section view of such a variable-focusliquid lens, formed in a dielectric enclosure 4 filled with a conductiveliquid 8. Dielectric 4 naturally has a low wetability with respect toconductive liquid 8. A lower surface of a wall of enclosure 4 includes ahollow 6, centered around an axis O perpendicular to this wall. Hollow 6is a truncated cone according to the present invention, such as thatshown in FIG. 5. A drop of an isolating liquid 2 is placed in hollow 6.As seen previously, isolating liquid drop 2 naturally takes a position Acentered on axis O. Isolating liquid 2 and conductive liquid 8 are bothtransparent, non-miscible, they have different optical indexes and havesubstantially the same density. The dioptre formed between liquids 8 and2 forms a surface of a liquid lens, the optical axis of which is axis Oand the other surface of which corresponds to the contact between thedrop and the bottom of the hollow. As electrode 10, including a hole 11in the vicinity of axis O, is placed on the external surface ofdielectric enclosure 4. As electrode 12 is in contact with conductiveliquid 8. Electrode 12 may be immersed in liquid 8, or be a conductivedeposition performed on an internal wall of enclosure 4. A voltagesource (not shown) enables applying a voltage V between electrodes 10and 12.

Voltage V may be increased from O volt to a maximum voltage, whichdepends on the used materials. When the voltage increases, isolatingliquid drop 2 deforms to reach a limiting position (designated withreference B). While drop 2 deforms from its position A to its positionB, the focus of the liquid lens varies.

It should be noted that, drop 2 being an isolating liquid, no microdropsoccur at its periphery when voltage V is high, conversely to what wouldoccur if the drop was a conductive liquid.

The conical shape of hollow 6 is such that, whatever the shape of drop 2that it contains, the curvature of its surface at any contact pointbetween the limit of the drop and the surface is smaller than that of atangent circle TC crossing this point. Thus, according to an aspect ofthe present invention, hollow 6 is such that, all along its deformationfrom its position A to its position B, liquid drop 2 is continuouslymaintained centered on axis O. A liquid lens with a accurately fixedoptical axis and with a focus varying with voltage V is thus available.

It should be noted that a hollow 6 according to the present invention,which ensures the continuous centering of liquid drop 2, is relativelysimple to implement and that it advantageously replaces theelectromagnetic centering means or the surface processing centeringmeans described in above-mentioned French patent application.

An A.C. voltage will preferably be used for voltage V, to avoid theaccumulation of electric loads across the thickness of material 4, fromthe surface on which is laid drop 2.

As an example, water charged with salts (mineral or others) or anyliquid, organic or not, which is conductive or made such by addition ofionic components may be used as a conductive liquid 8. For isolatingliquid 2, oil, an alkane or a mixture of alkanes, possibly halogenated,or any other isolating liquid non miscible with conductive liquid 8 maybe used. Dielectric wall 4 may be a glass plate or a superposition offluorinated polymer, epoxy resin, polyethylene. Electrode 10 may be ametal deposition.

FIG. 12 shows a simplified cross-section view of an alternativeembodiment of the variable-focus liquid lens of FIG. 11. Same referencesdesignate same elements in FIGS. 11 and 12. In this embodiment,electrode 10 may be a metal sheet in which hollow 6 is formed byembossing. It may also be a metal wall in which hollow 6 has been formedby machining, then polishing. Wall 4 then is, for example, a thintransparent plastic film flattened against electrode 10 and which covershole 11. This plastic film may for example be flattened bythermoforming.

In the example of application of FIG. 12, drop 2 has an idle diameter ofapproximately 1 to 5 mm. Conductive liquid 8 and the isolating liquid ofdrop 2 being substantially of same density, drop 2 has the shape of aspherical cap. When idle (position A), the edge of drop 2 makes an angleof approximately 45 degrees with the surface of hollow 6, if the latteris a cone having a 45-degree slope. In its limiting position (positionB), the edge of drop 2 makes an angle of approximately 90 degrees withthe surface of enclosure 4. The described device, using as a conductiveliquid 8 salt water having an optical index 1.35 and, as the isolatingliquid of drop 2, oil with optical index 1.45, enables obtainingapproximately 30 diopters of focus variation for an applied voltage of250 V and a dissipated electric power of a few mW. The frequency of theA.O. voltage ranges in this case between 100 and 10,000 Hz, its periodbeing much smaller than the system response time of approximately a fewhundredths of a second.

The variable-focus lens according to the present invention may have asize ranging between a few hundreds of μm and a few tens of mm and mayin particular be applied to the field of optoelectronic, endoscopy,imaging and vision systems.

Of course, the present invention is likely to have various alternativesand modifications which will occur to those skilled in the art. Inparticular, the present description has been made in relation withhollows having a circular cross-section, that is, formed by rotationaround an axis O. However, elongated hollows having the shape of achannel may for example be provided. In this case, FIGS. 5 to 7 will beconsidered as describing the cross-section view of a channel, and axis Owill represent the symmetry plane of said channel.

On the other hand, a hollow 6 with a flat bottom has been shown in FIG.11, which results in a plano-convex lens. Now, it has been seen that theshape of the bottom of hollow 6 does not influence its properties ofcentering of drop 2. Thus, biconvex or meniscus lenses may easily beformed by modifying the curvature of the bottom of hollow 6.

On the other hand, also, an embodiment of a variable-focus lens using aconical hollow such as in FIG. 5 has been shown in FIGS. 11 and 12, butthe present invention will easily be adapted to a variable-focus lensusing another shape of hollow according to the present invention.

Finally, a device including a network formed of groups of threeseparately-controlled variable-focus lenses, red-, green- andblue-colored, operating for example in all or nothing, enabling lettingthrough or stopping the light coming from a single white light sourcemay be formed, thus forming an illuminated color screen that can be verylarge and of low cost.

1. A method for centering a liquid drop (2) at a given location of asurface (4), comprising forming at this location a flared hollow (6)such that, at any point (CP1) of the contact limit between the drop andthe hollow, said hollow has a curvature smaller than or opposite to thatof a circle (TC) tangent to the hollow surface at said point and at asymmetrical point (CP2) of this surface.
 2. The method of claim 1,characterized in that the flared hollow (6) has the shape of a truncatedcone with an axis perpendicular to said surface.
 3. The method of claim1, characterized in that the flared hollow (6) has the shape of theupper central portion of a torus having an axis perpendicular to thesurface.
 4. A method for centering a liquid drop (14) on the externalsurface of a convex surface (16), comprising giving this surface at anypoint (CP1) of the contact limit with the drop a shape such that thissurface has a curvature greater than that of a circle (TC) tangent tothis surface at this point and at a symmetrical point (CP2) of thissurface.
 5. The method of claim 4, comprising forming the convex surface(6) by revolution against said axis of an arc of a circle having aradius smaller than that of said tangent circle.
 6. A variable-focuslens, including: a wall made of an insulating material (4), a drop of afirst, insulating, liquid (2) arranged on an area of a first surface ofthe wall, a second, conductive, liquid (8) covering the first surfaceand the drop, the first and second liquids being non-miscible, havingdifferent optical indexes and substantially the same density, and means(12) for applying an electric voltage (V) between the conductive liquidand an electrode (10) arranged on a second surface of said wall, whereinthe drop is placed in a flared hollow (6) of the wall according to themethod of claim
 1. 7. The variable-focus lens of claim 6, characterizedin that: the electrode (10) is a sheet metal, the flared hollow (6) is atruncated cone formed by embossing said sheet metal, centered on an axis(0) perpendicular to the first surface, and the bottom of which ispierced with a centered hole (11), and the insulating material wall (4)is a transparent plastic film flattened against the electrode and thewalls of the hollow, and which covers said hole.
 8. The variable-focuslens of claim 6, characterized in that: the electrode (10) is a sheetmetal, the flared hollow (6) is a truncated cone formed by 20 machiningsaid sheet metal, centered on an axis (0) perpendicular to the firstsurface, and the bottom of which is pierced with a centered hole (11),and the isolating material wall (4) is a transparent plastic filmflattened against the electrode and the walls of the hollow, and whichcovers said hole.
 9. A method for centering a liquid drop at a givenlocation of a surface, comprising the steps of forming at this locationa drop containment cavity surface such that, at any point (CP1) of thecontact limit between the drop and the drop containment cavity surface,said surface has a curvature smaller than or opposite to that of acircle (TC) tangent to the drop containment cavity surface at said pointand at a symmetrical point (CP2) of this surface.
 10. A method asclaimed in claim 9, wherein the drop containment cavity surface isrotationally symmetric.
 11. A variable-focus lens, comprising: a drop ofa first, insulating, liquid arranged in a drop containment cavity havinga sidewall with a first surface, a second, conductive, liquid coveringthe first drop, the first and second liquids being non-miscible, havingdifferent optical indexes and substantially the same density, and anelectric voltage source selectively connectable between the conductiveliquid and an electrode arranged on a second surface of said sidewall,wherein the configuration of the sidewall is such that, at any point(CP1) of a contact limit between the drop and the drop containmentcavity first surface, said surface has a curvature smaller than oropposite to that of a circle (TC) tangent to the drop containment cavityfirst surface at said point and at a symmetrical point (CP2) of thissurface.